Local streams receive plenty of abuse. Hit with pollution, storm
runoff, and deforestation, both next to the stream (called the
“riparian zone”) and within their watershed, we know that stream
ecosystems are not what they used to be. Stopping these stresses and
improving stream health will take the efforts of every resident,
business, and government agency within our county. Strong scientific
evidence supports the benefits of stream restoration, including
planting vegetation in the riparian zone. However, information about
the health of streams following restoration would be needed to help
prioritize the locations and designs of future planting efforts.

Fairfax County, the Fairfax County Park
Authority, and the nonprofit Earth Sangha have worked with volunteers
to revegetate nearly 30 stream riparian zones since April 2005. The
U.S. Geological Survey and NVSWCD are now monitoring and evaluating the
effectiveness of one of these riparian plantings in restoring stream
health. With the help of something called an “iButton”, a tiny
temperature monitoring device, we are evaluating the impact of riparian
zone restoration on stream temperature at Lake Fairfax Park in Reston.

The riparian restoration planting we evaluated was located just below
the dam at Lake Fairfax Park, in a formerly mowed area where the field
was too wet for recreation and mowing equipment was getting stuck in
the mud. After an enormous volunteer planting effort, the area around
the stream is now flourishing with wildlife and native plants like
ironweed and sycamore saplings. But stream health is more than having
the right plants and wildlife – it is also about having the right range
of water temperatures. Too hot, and the water doesn’t have enough
oxygen to support fish and insects. Hot water also speeds up some
chemical reactions and the breakdown of organic matter, which can lead
to a lack of food, too many nutrients, and oxygen deficiency.

Soon after
planting at Lake Fairfax Park, volunteers began monitoring stream-water
temperature. iButtons make collecting data on water temperature easy:
they are the size of a stack of four dimes, relatively inexpensive,
automatically store temperature at programmed intervals (every 30
minutes for us) over long time periods, and can be downloaded with a
Palm Pilot. About once a month, in hot, freezing, wet, windy, or
occasionally mild weather, the volunteers wade into the stream to
download data from the iButtons.

The stream at Lake Fairfax Park is a shallow and narrow
headwater stream that starts at a seep where ground water emerges from
a hillside and ends 200 yards later when it joins the larger Colvin
Run. From upstream to downstream, we’ve installed iButtons at five
locations within the stream: just downstream from the ground-water
seep; two sites in the currently forested riparian zone; in the
riparian planting area between the forest and mowed grass where a few
small trees, shrubs and tall grass now grow; and lastly, in a
completely open area where grass mowing continues. An iButton also
hangs off a tree to compare water and air temperature. Despite a few
data gaps due to ornery iButtons or losing an iButton in the muck, over
100,000 measurements now document the influence of our land management
on water temperature.

What is the effect of riparian vegetation on stream temperature? It
depends on the time of year. When the air is cold, the water
temperature also gets cold. In winter, the water got colder and colder
as it flowed downstream from the ground-water seep (which has a
relatively constant temperature), through the vegetated riparian areas
and into the unplanted open site. During the coldest weather, the
stream at the open, unplanted site froze solid for days, which rarely
happened at the planted site just upstream. Vegetation at the planted
site likely helped block the frigid winds and protect the stream.
Needless to say, a solidly frozen stream is not a good place for
wildlife, especially those that live in water!

But, in the summer, when it is hot, the water temperature got really,
really hot. Where there was no vegetation, the stream temperature got
extremely hot in the middle of the day in June, July, and August
compared to the planted and forested riparian sites. At times, the
stream water at the open site heated above 90ºF – that was 18ºF hotter
than at the planted site just upstream where tall grass, shrubs, and
trees shade the water.

Stream temperature at the five iButton monitoring locations from
September 2005 to November 2006. Temperature changes registered by
the iButton in the open, mowed field are more extreme than those
where vegetation is present. Air temperature is also shown to compare
with stream temperature.

Trees and shrubs help shade streams from solar heating during hot
weather and help insulate streams from freezing weather. The extremes
of our weather can lead to ‘fire and ice’ in streams without vegetated
riparian zones. This buffering effect of riparian plants on stream
temperature is crucial for maintaining good stream ecosystem health. Of
course, trees in the riparian zone also provide food and homes for
wildlife and help maintain water quality. As the trees grow and the
canopy fills in, we’ll continue to track stream temperature as evidence
of the importance of maintaining and restoring vegetation along our
streams.

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descriptive purposes only and does not imply endorsement by the U.S.
Government